New Dwarf Planet Sparks Debate about Planet Nine

The family of extended scattered disk objects beyond the classical Kuiper Belt just keeps getting bigger. The latest addition to this population of objects is a fairly substantial dwarf planet 700km across, currently referred to by the moniker 2015 RR245 (1). Its elliptical orbit is not absolutely defined as yet, but the best estimates give it an aphelion distance of about 120 Astronomical Units, and a closest approach to the Sun of about 34 AU (2). The Minor Planet Center describes the object as the 18th largest in the Kuiper Belt, but it is not yet clear what its surface features might include. 2015 RR245 takes approximately 700 years to orbit the Sun. 2015 RR245was discovered by National Research Council of Canada’s Dr J.J. Kavelaars while studying images taken by Canada–France–Hawaii Telescope in Hawaii in September 2015 .

Does this object fit in with Mike Brown’s analysis of the cluster of 6 (now 7) Sednoid objects which he argues (along with his dynamicist colleague Konstantin Batygin) point to the existence of a substantial planet beyond the Kuiper Belt (3)? Given the vague data regarding the orbit of 2015 RR245, it is perhaps too early to say. But other scientists are already citing the on-going discoveries of distant objects like 2015 RR245 as reasons to be cautious. In an informative on-line article, more nuanced than its title suggests, astrophysicist Ethan Siegel notes that the pattern of discovery of objects within and beyond the Kuiper Belt is subject to an observational bias favouring the closest objects. This means that the unknown populations of objects yet to be discovered may eventually statistically overwhelm the small populations of extended scattered disk objects and Sednoids already discovered. This, he argues, could bring Brown and Batygin’s analysis of the cluster into question.

Such an argument does not directly address the statistical fluke generated by the original clustering of six Sednoids, which were later joined in March 2016 by a seventh, uo3L91 (5). Instead, Siegel’s argument simply implies that this fluke could be overwhelmed by a much broader data set attained at a later point. But this is irrelevant, because 2015 RR245 actually enjoys a 9:2 resonance relationship with Neptune, and so is not a member of the same set of objects which has caused this controversy (6). Siegel’s argument is contested here by Dr Bannister:

“[Michele] Bannister [an astronomer at the University of Victoria in British Columbia] stirred up some buzz in March when she presented data about a different object in the far reaches of the solar system, known as uo3L91. At the time, Caltech astronomer Mike Brown – who was part of a team that discovered Eris back in 2005 – said the object’s far-flung orbit could serve as evidence for the existence of a super-distant super-Earth he dubbed “Planet Nine.” However, Bannister said 2015 RR245 has no connection to the case of uo3L91 or the search for Planet Nine.” (7)

It does seem, though, that each new object discovered beyond the Edgeworth-Kuiper Belt is going to generate this kind of debate: Does its presence help, hinder, or utterly fail to impact upon the potential existence of Planet Nine?

Another aspect to consider here is that the Caltech team seem to consider the population of these anomalous clustered objects to be a dynamic one, rather than a static collection held forever in thrall of Planet Nine. A newly-published on-line article by Konstantin Batygin traces the evolution of the thinking behind Planet Nine, and describes in some detail the rather odd dynamic patterns of resonance which the Sednoids have with his (and Dr Brown’s) proposed distant planet (9). It seems reasonable that larger-than-dwarf planets might be captured up in to extended scattered disk of Sednoids in this way, but what’s interesting is that Brown and Batygin envision a fluctuating population of Sednoids where Kuiper Belt Objects are pulled up into the scattered disk for a while, before falling back once more into the regular disk, to be replaced by others which experience similar perturbations:

“Surprising and unforeseen results continued to accrue. Upon a cursory examination of the simulation data, we noticed that gravitational torques exerted onto the Kuiper belt by Planet Nine would induce long-period oscillations in the perihelion distances of the confined KBOs. This naturally generated detached orbits, such as those of Sedna and 2012 VP113. Suddenly, the origins of these objects became abundantly clear: they are regular KBOs that have been pulled away from their original locations by Planet Nine. Moreover, the evolutionary calculations suggested that if we were to revisit the Kuiper belt in a hundred million years, objects like Sedna and VP would once again look like conventional, garden-variety KBOs, while some of the more typical objects would now be in detached orbits.” (9)

Again, this implies that the statistical criticism employed by Ethan Siegel is wide of the mark. It is what is going on in the specific clustering of anomalous objects that is crucial, not any buffering effect attributed to the broader populations around them – even those populations which remain undiscovered. And I’ll bet that those more distant undiscovered populations are even more irregular still than the Sednoids.